Recently, a factory plant growth system involving an artificial light source, which is unsusceptible to external factors including weather, climate and pest insects, has proceeded toward the practical use. The artificial light sources generally used include a high pressure sodium lamp, a low pressure sodium lamp, a xenon lamp, a metal halide lamp, a fluorescent lamp and a microwave lamp. Among of these, the high pressure sodium lamp, which has a relatively high luminous efficiency, is predominantly used.
However, in the artificial light source composed of the high pressure sodium lamp or the like, the balance between red light (having a wavelength region from 640 to 690 nm) and blue light (having a wavelength region from 420 to 470 nm), which is essential for photosynthesis phenomenon or the like, is not good, and therefore, the output thereof has to be increased significantly for wholesome plant growth. The low pressure sodium lamp has a higher luminous efficiency than the high pressure sodium lamp, but has a degraded light quality because it emits monochromatic light of sodium D line and is difficult to increase the output.
Such an artificial light source emits a high heat radiation and therefore has a significant load air-conditioning. In addition, the plant and the light source have to be sufficiently spaced apart from each other to prevent the plant from being affected by the heat radiation. Thus, there has been a problem in that the apparatus tends to be large.
In view of the problems, recently, the artificial light sources including a semiconductor optical device, such as a light emitting device (LED) and a semiconductor laser (LD), have been adopted. As for the semiconductor optical device, one having an emission wavelength region not including a heat ray can be adopted, so that the load of air-conditioning is reduced, and the whole apparatus can be made compact, so that the illumination efficiency is increased. Furthermore, the semiconductor optical device advantageously has a lifetime many times longer than that of the high pressure sodium lamp. In addition, since a lot of semiconductor optical devices are arranged in the form of a line or sheet, the configuration of the illuminator is readily and flexibly adapted to the plant to be illuminated, and the light emission density is readily controlled.
Another advantage of using the LED or LD is that intermittent illumination (pulse illumination) can readily provide the sunbeam spot phenomenon (intense sunshine filtering through foliage), which would occur in nature. During a short time when receiving the sunbeam spot, the plant stores the light energy in the form of a difference in proton concentration between inside and outside of the thylakoid statocyst of the chloroplast. Then, the light energy is used little by little to fix CO2. The CO2 absorption caused by the sunbeam spot is significantly higher than the CO2 absorption (proportional to (photosynthetic rate)×(time duration of sunbeam spot)), which is estimated based on the photosynthetic rate in a steady state with a continuous and constant light intensity. The difference between the CO2 absorptions becomes remarkable as the time duration of the sunbeam spot is reduced. That is, the efficiency of photosynthesis is higher when the plant is illuminated with the sunbeam spot for a short time than when it is continuously illuminated with light with a constant intensity.